1. Technical Field
The invention relates to methods of using bypass valves.
2. Description of Related Art
Vehicles consume significant amounts of fossil fuels. Due to limited resources and environmental pressures, vehicle manufacturers are attempting to reduce fuel consumption of the vehicles they manufacture while still providing sufficient power. Superchargers and turbochargers can be used to increase the power of an internal combustion engine. A particular system can include a combination of a supercharger, a turbocharger and turbo-compounding, using a hydraulic or mechanical continuously variable transmission to drive the turbocharger up to a specific speed or intake manifold pressure and then holding the speed to keep it at a desired boost pressure for the engine condition. Such a system relies on the supercharger to initially increase the boost pressure before the turbocharger can achieve sufficient boost pressure.
The application of Continuously Variable Transmissions (CVTs) to vehicular applications may also be used to improve fuel efficiencies by maintaining a variable but preferred drive ratio between an internal combustion engine and the driven load. In vehicular applications CVT drives may be used in the transmission drive line between the engine and the vehicle's wheels or in the drive line between the engine and various accessories. However applied, a CVT may be controlled by signals from a control module incorporating a software program that takes real-time output values from a number of sensors measuring various related parameters. The computed CVT control signals would then effect CVT ratio change by means of a servo actuator physically driving the input to the CVT's ratio-shift system.
Rolling traction CVTs rely on the “frictional” contact between rolling elements to transmit torque through their spinning input/output shafts. The force that effectively loads the contact surfaces between rolling elements within the CVT is often referred to as the normal force. The normal force allows the CVT to transmit tractive force (the torque being transmitted by the CVT). In rolling traction and similar CVTs, typically lubricated and cooled by traction fluid, the normal force is typically several times greater than the tractive force. The exact relationship between the normal and tractive forces varies with the specific design, operating conditions, temperature and type of traction fluid used. If the ratio of normal force to tractive force is too low, then slip may occur and damage to the CVT's precision tractive surfaces may result. If the ratio of normal force to tractive force is too high, then the CVT's efficiency may be compromised and the CVT's service life will be disproportionally reduced.
The ratio of normal to tractive forces may vary significantly in practice due to a variety of torque variations. For example, the torque pulsations caused by the firing impulses of an internal combustion engine create cyclic angular accelerations on the rotating crankshaft. While the output from an internal combustion engine is usually specified in terms of mean torque, the pulse peaks may be several times the mean torque value. These variations can create temporary conditions where the ratio of normal to torque forces in a connected CVT may be too low or high, creating the issues described above.
Other types of torque variation conditions often occur during the operation of an engine. For example, in the case of an engine incorporating a supercharger driven from the engine's crankshaft, a sudden demand by the driver for a high power output for brisk acceleration would generate a sudden increase in torque as the supercharger is engaged. Due to these types of torque variations, parts in the driveline are often sturdily designed to handle these types of variations. In addition, a variety of types of damping mechanisms have been developed to help manage these variations.
These torque variations have implications to the use of CVTs or other types of transmissions or accessories such as superchargers and turbochargers with internal combustion or reciprocating engines in general.